Core and coil ballasts can express certain very rare modes of failure wherein the coils heat up rapidly due to excessive currents. In some instances, the temperatures can become so high that the ballast insulation smokes or ignites. Although these malfunctions are very uncommon, they should be avoided whenever possible since they can potentially cause serious personal injury or property damage.
Conventional automatically resettable thermal protection, if built into ballasts, has not proved effective because the temperature of the coils can rise to destructive levels so quickly that the automatically resettable thermal protectors cannot react in time. Automatically resettable thermal protectors also allow destructive operating temperatures to potentially reoccur many times because the system will automatically reset once the protection circuit has removed power and the system has had time to cool. While one such reoccurrence may not result in a catastrophic failure, many such cycles can almost be guaranteed to induce a failure that is likely benign, but possibly violent in nature.
Non-resettable types of thermal protectors, based on melting wax, or other materials, have been used in transformers, which are similar in character to ballasts. Such melting-material thermal protectors are known to degrade over time. Thus, failures are common at safe operating temperatures because a downward shift occurs from the initial temperature trip-point over time.
High intensity discharge (HID) lighting is a growing sector in the lighting industry. Initially HID lighting systems were predominantly used for outdoor applications. Now they are being increasingly used for more and more indoor applications. While a violent end to a ballast's life may not present a significant hazard when it occurs in outdoor applications, the potential for damage is increased when the system is used in connection with indoor applications. In many cases, the possibility of a violent end of life failure, though being very small, makes usage of HID lighting systems prohibitive for some critical indoor applications. Having comprehensive ballast protection will open those new applications to HID lighting.
Therefore, it is an object of present invention to provide an open core and coil ballast with improved features that ensure effective prevention of non-passive failure at the ballast end of life.
It is another object of the present invention to provide a ballast that effectively limits operating temperatures and prevents catastrophic temperatures and consequential non-passive failures in the lighting systems.